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This document outlines the format for human-readable IDs within matrix.
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Overview
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--------
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UTF-8 is quickly becoming the standard character encoding set on the web. As
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such, Matrix requires that all strings MUST be encoded as UTF-8. However,
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using Unicode as the character set for human-readable IDs is troublesome. There
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are many different characters which appear identical to each other, but would
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identify different users. In addition, there are non-printable characters which
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cannot be rendered by the end-user. This opens up a security vulnerability with
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phishing/spoofing of IDs, commonly known as a homograph attack.
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Web browers encountered this problem when International Domain Names were
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introduced. A variety of checks were put in place in order to protect users. If
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an address failed the check, the raw punycode would be displayed to disambiguate
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the address. Similar checks are performed by home servers in Matrix. However,
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Matrix does not use punycode representations, and so does not show raw punycode
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on a failed check. Instead, home servers must outright reject these misleading
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IDs.
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Types of human-readable IDs
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---------------------------
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There are two main human-readable IDs in question:
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- Room aliases
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- User IDs
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Room aliases look like ``#localpart:domain``. These aliases point to opaque
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non human-readable room IDs. These pointers can change, so there is already an
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issue present with the same ID pointing to a different destination at a later
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date.
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User IDs look like ``@localpart:domain``. These represent actual end-users, and
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unlike room aliases, there is no layer of indirection. This presents a much
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greater concern with homograph attacks.
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Checks
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------
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- Similar to web browsers.
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- blacklisted chars (e.g. non-printable characters)
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- mix of language sets from 'preferred' language not allowed.
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- Language sets from CLDR dataset.
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- Treated in segments (localpart, domain)
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- Additional restrictions for ease of processing IDs.
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- Room alias localparts MUST NOT have ``#`` or ``:``.
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- User ID localparts MUST NOT have ``@`` or ``:``.
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Rejecting
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---------
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- Home servers MUST reject room aliases which do not pass the check, both on
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GETs and PUTs.
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- Home servers MUST reject user ID localparts which do not pass the check, both
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on creation and on events.
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- Any home server whose domain does not pass this check, MUST use their punycode
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domain name instead of the IDN, to prevent other home servers rejecting you.
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- Error code is ``M_FAILED_HUMAN_ID_CHECK``. (generic enough for both failing
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due to homograph attacks, and failing due to including ``:`` s, etc)
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- Error message MAY go into further information about which characters were
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rejected and why.
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- Error message SHOULD contain a ``failed_keys`` key which contains an array
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of strings which represent the keys which failed the check e.g::
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failed_keys: [ user_id, room_alias ]
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Other considerations
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--------------------
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- Basic security: Informational key on the event attached by HS to say "unsafe
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ID". Problem: clients can just ignore it, and since it will appear only very
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rarely, easy to forget when implementing clients.
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- Moderate security: Requires client handshake. Forces clients to implement
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a check, else they cannot communicate with the misleading ID. However, this is
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extra overhead in both client implementations and round-trips.
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- High security: Outright rejection of the ID at the point of creation /
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receiving event. Point of creation rejection is preferable to avoid the ID
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entering the system in the first place. However, malicious HSes can just allow
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the ID. Hence, other home servers must reject them if they see them in events.
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Client never sees the problem ID, provided the HS is correctly implemented.
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- High security decided; client doesn't need to worry about it, no additional
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protocol complexity aside from rejection of an event. |